ibuf: describe why ibuf_*() functions are copied

I wrote it mainly to clarify when the hack could be removed.

Follows up #59

HACKING.md

@@ -0,0 +1,18 @@
+# Hacking
+
+Here we collect arcane knowledge, which may be useful for developers of the
+module.
+
+## memcached_ibuf
+
+`third_party/memcached_ibuf.[ch]` is the copy of ibuf from the small library
+with renaming of the functions. The motivation is to avoid possible name clash
+between tarantool's symbols and the module's symbols. See [1] and [2] for
+details.
+
+We can remove it, when we'll dedice to drop support of tarantool versions
+affected by [1] or will find another way to overcome the name clash (see [3]).
+
+[1]: https://github.com/tarantool/tarantool/issues/6873
+[2]: https://github.com/tarantool/memcached/issues/59#issuecomment-1081106140
+[3]: https://github.com/tarantool/memcached/issues/92#issuecomment-1081128938

README.md

@@ -190,3 +190,8 @@ For custom configuration file path, please, use `SASL_CONF_PATH` environment var
 ## Caution
 
 This rock is in early beta.
+
+## Hacking
+
+We're collecting information for the module developers in the
+[HACKING.md](HACKING.md) file.